Biomechanical Properties of the

Cornea in Normal-Tension Glaucoma

Authors:Leonidas Traipe

Ines CayuqueoFabiola Cerfogli

Claudia GoyaAllister Gibbons

Universidad de Chile.Fundación Oftalmológica los

Andes.

The authors have no financial interest

Purpose/Objectives

• To evaluate corneal Hysteresis (CH), Corneal resistance factor (CRF), Corneal Compensated IOP (ccIOP) and Goldman type IOP (gIOP), in patients with the diagnosis of normal-tension glaucoma.

Materials/Methods• We analyzed 49 eyes of 26 patients seen

in our center. 22 (84.6%) of patients were female. The total sample had on average 68 ± 9.9 years.

• We identified the patients through the retrospective revision of clinical charts, and posteriorly examined them between March and July of 2008.

• The patients were examined with the Reichert Ocular Response Analyzer and Ultrasonic Pachimetry (CCT).

Inclusion Criteria• Confirmed Diagnosis of Normal-Tension Glaucoma

(NTG)• IOP < 21 mmHg with the Goldman aplanation

Tonometer in all examinations without treatment• Open Angle determined by gonioscopy• Typicall glaucomatous optic disc damage, excavation

size, notch, etc.• Visual Field Defect compatible with the Optic Nerve

Defect• No previous intraocular surgery

Exclusion Criteria• IOP > 21 mmHg with the Goldman aplanation

Tonometer in any examination without treatment• Corneal ectasia• Contact lens user• Pregnancy

Results / CH

R = 0.763p<0.0001

R = 0.554p<0.0001

Results / CRF

R = 0.649p <0.0001

R = 0.761p <0.0001

• Practically no correlation (r = 0.407).

• This means aproximately only 16.6% of ccIOP could be explained by the CRF (R2 = 0,166)

R = 0.407p < 0.004

Results / gIOP

R = 0.902p <0.0001

R = 0.434p < 0.002

• ccIOP and CCT do not seem to be governed by a linear relantionship. Either:– There is no relationship– The relationship is more complex

R = 0.182p < 0.19

• ccIOP was significantly higher than the gIOP.

The average difference was 2.01 mmHg

Conclusions

• In our sample of NTG patients, Corneal Compensated IOP was higher than goldman simulated IOP - Approximately 2 mmHg.

• Corneal Resistance Factor:• Correlated positively with gIOP (R=0,76)• Correlated poorly with ccIOP (R=0,41)

• There was a strong correlation between Corneal Hysteresis and Corneal Resistance Factor.

• In our study population, our results differed from expected in that:– We found some correlation between Corneal

Hysteresis and Corneal Resistance Factor with the Central Corneal Thickness.

– There seems to be none or poor correlation between gIOP or ccIOP with Central Corneal Thickness.

• This could be explained in part because:– We used a diseased population.– We studied a population (chilean) for which

we have no normal values calculated.

Conclusions